Method of and means for separating desired from undesired electric currents of different frequencies



prll 28, 1931. E, G, GAGE 1,802,760

METHOD 0F AND IEANS FOR SEPARATING DESIRED FROM UNDESIRED ELECTRIC CURRENTS 0F DIFFERENT FREQUENCIES Filed Nov. 2. 1927 3 Sheets-Sheet 2 MM INVENTOR April 28, 19341.

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UNITED STATES PATENT OFFICE EDWARD G. GAGE, OF BROOKLYN, NEW YORK, ASSIGNOR TO RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., A CORPORATION 0F DELAWARE METHOD OF AND MEANS FOR SEPARATING DESIRED FROM UNDESIRED ELECTRIC CURRENTS OF DIFFERENT FREQUENCIES Application filed November 2, 1927. Serial No. 230,421.

This invention relates to certain improvements in separating desired from undesired electric currents of different frequencies and especially in separating such currents in telegraphy, telephony, and television or telephotography, whether by wire or radio.

The principle employed may be called for convenience frequency divergence.

It is well-known that high frequency currents such as employed in the above-named systems are capable of separation by tuning to certain frequencies to the exclusion of other frequencies. v

It is also well-known that there is a practical limit to the extent to which this may be carried out and that if the desired and undesired frequencies approach too closely in similarity of frequency they cannot be separated by ordinary tuning means.

An object of my invention is to separate such currents by a method which permits such great' selectivity that if the desired currents differ at all in frequency from the undesired, regardless of how small this difference is, they can be separated provided the process be carried far enough.

v Other and ancillary objects will be apparent to those skilled in the art.

In carrying out my invention I provide means interposed between the collector or line of a radio or wire system and the receiver by which the received desired and undesired currents are caused to pass through a progressive series of stages or steps of amplification in such manner that the algebraic sum of the frequencies of the desired currents at the end of the series remains unchanged or is equal to the original frequency whilst the undesired'frequencies occurring at dierent stages of the series becomes changed by a predetermined progression and a divergent series is formed, the resultant or algebraic sum differing from the original depending upon the number of steps or stages employed.

In carrying out my invention in forms illustrated in the accompanying drawings, I provide a series of stages or steps of vacuum tube amplifiers and detectors which are continually repeated by duplicates of the original stages connected in series.

One of the units in the series may be called for convenience a frequency reducer, in that it reduces the original frequencies and consists of a radio frequency amplifier, the output currents of which are heterodyned by a local source of oscillations.

The other unit in the series may be called for convenience a frequency increaser, the purpose of Which is to double the frequency of the output currents and consists of a vacuum tube amplifier, the output currents of which are doubled by means of full-wave rectification by suitable rectifiers.

The mathematical principle on which the method depends, consists in selecting a local heterodyne frequency which will reduce the received frequency to a value exactly onehalf of the desired, and then restoring the desired currents to the original value by passing them through the frequency doubler which doubles the frequency previously halved, and repeating the process through as many such stages as required.` Undesir-ed frequencies differing ever so slightly from the desired may be eliminated if a sufficient number of stages are employed, since they will be thrown more and more off tune by each succeeding pair of stages while the desired frequency remains unchanged as to frequency but greatly amplifiedas t'o signal strength.

A feature of the invention relates to a method for causing the desired current or signal to .reappear at definite intervals throughout the receiving network, with progressively increasing amplitude, whereas the undesired currents never reappear after their first reception except in progressively diminishing amplitude. y

Another feature of the invention relates to the simplification of the complicated circuits necessary for the operation of the system.4

Other features of the invention will appear from the following description taken in connection with the drawings, of which:

Fig. l shows the series of stages of the Ieceiving circuit employed.

Fig. 2 shows in detail the circuits of the various steps or stages employed in the system.

Fig. 3 shows curves of amplitude divergence of desired and undesired currents.

Fig. 4 shows curves of frequency divergence of desired and undesired currents.

In Fig. 1, numeral 1 indicates an antenna which may also be the collector of a line wire system. Currents from the antenna or collector are fed into the first step in the series, designated as numeral 3. As shown in the drawing the respective frequencies received by the collector are shown as 100,000 and 100,001 cycles. The former is taken as the frequency of the desired currents and corresponds to a radio wave length of 3,000 meters. The latter is taken as an undesired frequency and corresponds to a wave length of 2,999.9 meters.

Itis to be understood that these frequencies are merely chosen as convenient examples, any combination of frequencies being capable of being used.

It is to be noted that at the beginning of the series, with one step of tuning only, the frequency difference between desired and undesired currents is 1 cycle and it is to be noted how this difference between desired and undesired frequencies increases as additional steps are added.

The step 3 is directly connected with the receiving antenna or collector which also receives from a local oscillator, 2. rThis local oscillator consists of a thermionic generator adjusted to exactly one-half the frequency of the desired currents, in the present in stance 50,000 cycles, and is preferably radiated from a very small non-directional antenna adjusted to include within its range all the loops of the series, namely 6, 9 and 12, and also the antenna 1.

These loops are preferably adjusted at such an angle that they are at right angles or noninductive relation to each other but still able to receive from the common oscillator. This is accomplished by placing the loops in noninductive relation to each other in one plane but in radiation relation to the oscillator, which is on a plane partly common to all loops. The disposition of the loops is indicated in Fig. 1.

The purpose of this disposition is to prevent as much as possible, interference between stages. This is bound to occur to a certain extent, although the desired signal always predominates because it is more often in tune while all other currents are detuned. Harmful regenerative effects are most to be guarded against.

The steps 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 18 are all entirely encased in metal boxes or shields with the leads from one step to another encased in metallic cable or tubing. Both the shielding boxes and cable sheaths are well-grounded for further prevention of interference between stages. The grounded shield, 19, may take the form of a sheet-metal housing over the tops and sides of the apparatus as a whole, leaving one side open for loop reception, this side being out of radiation line of the antenna. Filament batteries may be grounded when necessary.

Both frequencies of 100,000 and 100,001 cycles as received on the antenna or collector are heterodyned by the frequency of 50,000 cycles from the local oscillator, 2, of 50,000 cycles. The results of this heterodyning are two new frequencies, 50,000 and 50,001, these being the differences respectively between the local and the desired and undesired currents. Desired frequencies are equal heterodyned.

These frequencies are now amplified by thermionic amplifiers and passed to the frequency increaser or doubler, 5. It is not essential in all cases to include an amplifier with each stage but it facilitates locating out-of-tune steps. With more than three stages it is preferable to also employ retarding circuits with each amplifier to prevent dephasing effects. Each stage is tuned to desired currents.

The frequency doubler, 5, is a full-wave rectifier such as Zennecks doubler as explained in Zennecks book Wireless Telegraphy, first edition, 1915, published by McGraw Hill Co., N. Y., translated by A. E. Seelig-see pages 379, 880, 342 and 427, consisting of preferably thermionic rectifiers arranged in the well-known manner for frequency doubling.

The result of passing the frequencies of 50,000 and 50,001 cycles through the frequency doubler, 5, is to create a new set of frequencies, namely, 100,000 and 100,002 cycles.

From step 5 t-he frequency-doubled currents are led to step 6 which is similar to step 3 being tuned and aimed to receive on its loop, a frequency of 50,000 cycles. It will of course also receive the frequency of 100,000 and 100,002 cycles from the previous step.

These three frequencies produce a frequency-resultant of 50,000 cycles, obtained by heterodyning 100,000 cycles with 50,000 cycles and also a frequency of 50,002 cycles which is obtained by heterodyning 100,002

cycles from the same local oscillator of 50,000 cycles. Harmonics in great numbers also appear but in lesser amplitudes.

These latter frequencies of 50,000 and 50,002 cycles are now passed through a second frequency doubler, 8, and emerge as 100,000 and 100,004 cycles respectively for desired and undesired currents.

The cycle of operation is then repeated through as many stages as desired, when the llO final set of frequencies is detected by a thermionic or other detector, 16, and amplified by audio-frequency amplifier, 17, where the amplified currents are utilized in the consumption circuit, 18, which may for example be associated with the stylus of a telephoto or television system.

It is at once apparent that the desired frequency of 100,000 cycles has repeated itself and is still 100,000 cycles while the undesired frequency has never been repeated but is further away from the desired frequency than before, being now 100,004 cycles.

In Fig. 2, numeral 20 indicates a tuning transformer of the usual type with associated variable condenser, 21, for tuning the input circuit of the radio frequency amplifier, 22, having plate battery, 23, grid-biasing battery, 24, tuning inductance and variable resistance, 25. Numeral 26 indicates the metal box shield and 27 the shielded cable or tube. Each tube in the series is individually supplied with the usual filament battery.

The output circuit of this stage is fed into the input circuit of radio frequency amplifier, 30, having likewise a tuning transformer, 28, variable condenser, 29, plate battery, 31, grid battery, 32, tuning inductance, 33, and variable resistance, 34. Numeral 35 indicates the metal box shield for connecting cable.

The output circuit of this amplier passes the amplified received currents into the input circuit, 36, of the frequency doubler step, of which 37 and 38 are respectively thermionic rectifying tubes of a full-wave rectifying system such as that of Zenneck.

In th-e secondary, 40, of transformer, 39, is located a condenser, 41, for adjusting the frequency of the full-wave rectified currents, which is transferred to, the input circuit of the next step, 42 indicating the usual shield. Numeral 43 indicates the tuning condenser of the input circuit of this step which is that of the radio frequency amplifier tube, 44, with customary grid leak and biasing battery, after which the same cycle of operation is repeated.

The loop, 45, of the second step is adjusted to receive from the local oscillator. It is not possible or necessary to have all loops at maximum receptivity as to direction, but they should maintain their non-inductive relation to each other as much as possible.

Plate battery, '46, adjustable inductance, 47, and adjustable resistance, 48, of this step complete the equipment of the second step, the loop being connected to Igrid and filament of the amplifier in the usual way.

As many stages as desir-ed may now be added, after which the desired and undesired currents are detected by detector, 52, preferably of the thermionic type with its `associated input transformer, 50, and tuning condenser, 51, plate battery, 53, and grid leak condenser,

54, and shielding box, 55, from which the detected currents are led to the audio-frequency amplifier, 57, with associated audio-frequency transformer, 56, and plate battery, 58, with shield, 59. Finally these amplified currents are fed through an audio-transformer, 60, to a rectifier, 61, of thermionic or gaseous type, including a stopping condenser and relay device in its circuit. A condenser may shunt the primary of 56.

The final consumption circuit which may include a relay armature, 62, may consist of the stylus of a television or facsimile telegraph apparatus or other work circuit, 63 the entire mechanism preferably being shielded by the box shield, 64.

In Fig. 3, curve 65 indicates for representation only the strength of the desired signal after passing through a given number of stages compared to the strength of the undesired signal 66 after passing through the same number of steps, numeral 67 indicating energy divergence.

In Fig. 4, curv-e 68 indicates for representation the frequencies of specific desired currents after passing through a given number of stages. 69 indicates the frequency of specific undesired currents after passing through the same number of stages. 70 indicates frequency divergence. i

It will be found that after the first heterodyning between the local oscillator, 2, and the first step, 3, a gradually increasing number of frequencies will appear until after a few steps have been included, there will be present many frequencies seemingly of all possible values. This is to be expected and can be prevented to a certain extent by the use of high and low pass filters between each stage, but in view of the fact that there is always one predominating frequency as to amplitude, namely that of the desired, the use of filters is optional.

It would be preferable to neutralize each sta-ge of radio-frequency amplification in the manner of Rice or I-Iazeltine, but practical difficulties in taking care of so many adjustments require a more simple scheme. This 1 is found in the ordinary energy-reduction method of inserting a variable resistance in each amplifier output to cut down the gain when excessive. Some means to prevent inter-action to the point of oscillation between stages must be employed.

The final frequency of the desired current depends on the number of stages employed, and is quite easily made audible at an effective frequency by heterodyne selection ofv suitable value.

Where two signals of seemingly identical frequencies are encountered, a noticeable difference is apparent after the first stage, thereafter becoming more noticeable. with each additional stage.

In the case of separating desired signals from strays, the same procedure is followed as for separating signals of small frequency difference, with the exception that current limiters should always be used before the first stage. The system is not as well adapted to stray elimination as to communication channel increasing, owing to the large number of steps necessary to eliminate currents of high decrement, which are not easily detuned.

The procedure for carrier wave communication over line wires is substantially the same as for radio communication, although the frequencies employed differ.

It is of the utmost importance that means for accurate tuning be provided for all stages, and single control for all stages if employed must be designed with greater care than for ordinary radio reception.

Tuning condensers having small loss and small zero capacity are to be preferred and are preferably controlled by long levers. Extreme care must be taken in separating currents of apparently the same frequency and all the precautions employed with the most sensitive regenerativecircuits as regards body capacity effects are necessary.

I am aware of systems employing frequency doublers for the purpose of greater selectivity in reception.

In particular I am aware that Alexanderson in the United States and Scott-Taggart in England have proposed such systems, and I do not claim the features exhibited therein. Such systems make use of the comparative selectivity of high and low frequencies only. In the present invention the selectivity is obtained by the periodic recurrence of the eXact frequency of the desired currents but not of the undesired simultaneously.

While the description herein is confined to specific apparatus it will be apparent to those skilled in the art that many variations may be made without departing from the scope of the appended claims.

I claim:

l. The method of separating desired from undesired electric currents of different frequencies which consists in receiving and amplifying vsaid currents'iiwsisiof stages and iproducing during said amplification a progressively increasing @frequency divergenere between desired and undesired curundesired electric currents of different frequencies which consists in receiving and amplifying said currents in a series of stages, in causing the frequency of the desired currents to reappear periodically at intervals throughout the series of stages, and in causing the frequency of the undesired currents to reappear as a progressively changing frequency throughout the series of stages.

3. The method of separating desired from undesired electric currents of dierent frequencies which consists in receiving and amplifying said currents in a series of stages, 1n causing the desired currents to reappear periodically throughout the series of stages with progressively increasing amplitude, and in causing the undesired currents to appear at every stage with progressively changing frequency and progressively diminishing amplitude.

4l. The method of separating desired from undesired electric currents of different frequencies which consists in receiving and amplifying both desired and undesired currents,

heterodyning both said desired and undesired frequency currents by a frequency equal to one-half that of the desired frequency, doubling the resultant frequencies, and select-ing for use as the desired frequency that frequency which has the same frequency as the original frequency of the desired current.

5. Means for separating desired from undesired electric currents of different frequencies comprising a series of stages of amplification, means for heterodyning both desired and undesired currents in each stage from a source of frequency one-half that of the desired current, means for doubling the resultant frequencies and means for producing from the resultant for use as desired current, that frequency which is the same frequency as the initial frequency of the desired current.

G. The method of separating desired from undesired electric currents of different frequencles which consists in receiving said currents in a series of stages and in causing the ratio of the desired to undesired frequencies to progressively increase at each stage.

7. The method of separating desired from undesired electric currents of different frequencies which consists in receiving said currents in a series of stages and in causing the ratio of the desired to undesired frequencies to progressively increase at each stage both as to frequency and as to intensity of energy.

8. In a radio frequency amplifying system, a series of amplifiers, tuning means therefor, a frequency reducer interposed between two amplifiers and a frequency multiplier coupled in said series.

9. In a radio frequency amplifying system, aseries of amplifiers, tuning means therefor, a frequency reducer interposed between two amplifiers and a frequency multiplier connected with said series, said frequency multiplier resultant being substantially equal to said frequency reducer resultant for the desired frequencies but not for the undesired.

10. Means for separating desired from undesired frequencies comprising a series of alternate stages of frequency reducing means and multiplying means, the resultant of the series being a greater percentage difference for the undesired than for the desired frequencies.

11. In aradiopfrequency amplifying system, a ,series of amplifiers having tuning means tlerefo""forr'reciving signals of desired frequencies accompanied by signals of undesired frequencies, a local oscillator, means vforvheterodyning111m of amplirers both said desired and undesired frequency energy, andgneansrfor causing increasingseparationsbetweenusaid desired and said undesired frequency energy during successiveramplifying stages, said means including means for doubling the resultant hetero- ,15 "'dyned beat-notes in each of said stages.

,l 12. In a system for separating desired from undesired electric currents of different frequencies, means for receiving both desired and undesired signals, means for hetrodyning both said frequencies by a frequency equal to half of the desired frequency, means for doubling the resultant frequencies, means for again heterodyning the doubled frequencies resulting from said first het-erodyning and multiplying for producing beat frequencies,

and means for multiplying the resultant beat frequencies and for producing increasing separation between desired and undesired frequency energy.

13. The step in the method of separating desired from undesired electric currents of different frequencies during amplication thereof which consists in producing during the amplification a progressively increasing frequency divergence between the desired and the undesired frequency currents.

14. The step in the method of separating desired from undesired electric currents of different frequencies during amplification which consists in causing the frequency of the undesired currents to reappear during4 the amplification as a progressively changing frequency.

15. The steps in the method of separating desired from undesired electric currents of dierent frequencies during a series of amplifcation stages thereof which consists in causing the frequency of the desired currents to reappear eriodically at intervals throughout the amp ification stages, and in causing the frequency of the undesired currents to reappear as a progressively changing frequency during the amplification stages.

16. The step in the method of separating desired from undesired currents of different frequencies during signal repetition which consists in causing the frequency of undesired currents to reappear during the signal repetition as a progressively changing frequency.

Signed at New York in the county of NewA York and State of New York this 1st day of i November, A. D. 1927.

EDWARD G. GAGE. 

